For example, JP2014-107916A discloses an electric motor, in which a stator core is axially held between two frame ends. These frame ends are fixed together by fixture elements, such as bolts, to eliminate a case that covers an outer side of the stator core. The number of the components and the number of assembling steps can be advantageously reduced by eliminating the case.
In the case where the above electric motor is used as the drive source of the power steering apparatus of the vehicle, it may not be required to provide a complete waterproof function depending on an installation location of the electric motor in the vehicle. Therefore, it is desirable to manufacture the electric motor at lower costs by simplifying the structure of the electric motor through elimination of the waterproof function. However, although the waterproof function may not be required, it is required to have a drip-proof function to limit intrusion of water into an inside of the electric motor in a case where the water adheres to the electric motor due to, for example, dew condensation.
In the electric motor of JP2014-107916A, the intrusion of the water into the inside of the electric motor may be significantly limited by simply closing a hole formed in the frame end. However, the frame end is fitted to the outer side of the stator core. Therefore, in a case where the electric motor is installed in the vehicle in such a manner that an axis of a rotatable shaft of the electric motor extends in a direction that is parallel to or slightly tilted relative to a direction of gravitational force, water tends to be accumulated at a corner between a fitting portion of the lower frame end, which is located on the lower side in the direction of gravitational force, and the stator core. Therefore, although the water may not immediately penetrate into the inside of the electric motor, the water may disadvantageously penetrate into the inside of the electric motor through a boundary between adjacent two components of the electric motor when expansion and contraction of the components caused by a temperature change are repeated.
In order to address the above disadvantage, the inventor of the present application has proposed to provide a tubular portion, which has a maximum outer diameter smaller than a maximum outer diameter of the stator core and contacts the stator core, to the lower frame end located at the lower side in the direction of gravitational force upon installation of the electric motor to the vehicle. In this way, the water, which flows along an outer wall surface of the stator core, is less likely to be accumulated at a corner between the stator core and the tubular portion. However, a contact location (the tubular portion) between the lower frame end and the stator core is radially inwardly displaced from the fixation locations of the bolts at the lower frame end. Thereby, the frame end may be flexed (or warped) by the axial force of the bolt, so that a gap is formed between the frame end and the stator core, and the water may possibly enter the inside of the electric motor through the gap.